1. Field of the Invention
The present invention relates to laser diode arrays. More specifically, the present invention relates to structures and methods for mounting laser diode arrays.
2. The Prior Art
Most laser diodes, particularly those operating at high power are typically mounted by soldering the laser diode or laser diode array to a heat spreader formed from a thermally conductive material such as diamond or copper. The heat spreader is itself fixed to a sub-mount formed from a thermally conductive material such as copper. This final assembly is attached to a finned heat sink or a thermal electric cooler. The sizes and shapes of the various pieces and overall assembly can vary depending on the application.
Most of the design considerations for a particular mounting scheme are as much concerned with ease of assembly as with efficiency of heat removal. Because only the D.C. electrical characteristics and not light output of an individual laser diode can be tested in wafer form, individual laser diodes are typically mounted on the heat spreader and/or sub-mount and are then tested before committing to final assembly. This two-step assembly procedure is important in order to maintain acceptable yield. Rework, necessitated by a poor or inoperative individual laser diode in a laser diode array, is expensive after final assembly; thus the mounting components and procedures are designed so that the laser can be tested optically as soon as possible and subsequent handling of the laser itself is minimized. Relative and absolute thermal expansion of the parts is important in applications such as fiber-optic communications where alignment and stability is important.
For high power applications, such as diode-pumped YAG lasers requiring concentrated high intensity laser radiation sources, such as laser diode arrays which may have many (50-100) emitting regions in a single 1 cm by 0.08 cm bar, a "rack and stack" approach is often employed which is extremely effective for removing high heat flux.
The laser diode array is soldered to a heat spreader which carries the heat from the device to a cooled backplane which acts as the heat sink removing the heat. The basic rack and stack unit is repeated by stacking units one above another, resulting in a very compact concentrated high power laser light source with very high heat removal capability. Sometimes an insulating spacer is mounted on top of the heat spreader to aid in assembly and to act as a wire bonding pad. As discussed above with the conventional mount, the laser diode array is first attached to a heat spreader or sub-mount which facilitates optical testing before final assembly. Once the laser diode array optical performance is confirmed, the bars are then attached to the heat sink, a difficult process when many bars are stacked on top of one another. In addition, since electrical current passes top to bottom through the laser diode array, care must be taken not to electrically short the laser diode array and to isolate it properly from the other laser diode arrays and the heat sink.
Because the laser diode arrays used in these applications dissipate large amounts of heat into small areas, the heat spreader must have good thermal properties as well as allow easy test and mounting. The heat sinks or cooled backplanes to which the mounted laser diode array is attached must be able to remove large quantities of heat from a small area. The advanced heat sinks used in these applications are typically impingement coolers or microchannel coolers. The impingement cooler sprays a cooling fluid directly against the backplane and is then recirculated through another cooling stage. A microchannel cooler, shown in the schematic above, circulates a fluid through narrow channels etched in a material such as silicon. Properly designed, these devices can handle considerable heat fluxes.
In one instance, developed by Lawrence Livermore Laboratory, a rack and stack mounting scheme has been developed where the heat sink to which the laser diode array is attached is itself a microchannel cooler. Thus in the rack and stack embodiment, laser diode arrays are mounted on individual microchannel coolers and this assembly stacked up. This requires solving complex design problems on how to get high pressure cooling fluid efficiently to all the coolers and how to channel electrical current through the coolers themselves.
In view of the state of the art, there exists a need for providing a structure and method for mounting an array of laser diode emitters which overcomes the shortcomings of the prior art, which include difficulty of assembly leading to high cost and low yield, and the difficulty with working with high thermal conductivity materials to form rack and stack components.
It is an object of the present invention to improve heat removal from laser diode arrays.
It is another object of the present invention to reduce laser diode array assembly complexity and to reduce yield losses during laser diode array assembly.
It is a further object of the present invention to improve mechanical alignment precision of laser diode arrays, and to reduce the complexity of power supply metallization patterning for laser diode arrays.